The invention relates to Vee bottom planing boat hulls, and more particularly to controllably variable hydrodynamic planing surface configurations.
The invention arose during development efforts directed toward active hull planing surface design to improve the overall efficiency of the boat as it operates over its speed, weight, and center of gravity range. In one such design effort, this is done by forming a notch in the boat bottom by putting a step forward of the transom, to improve the trim angle of the boat at high speeds, and then using an integrated trim tab to provide lift at the rear of the boat to limit the boat's trim angle at lower speeds.
In one embodiment, optimum planing efficiency occurs at a boat trim angle of roughly 4.5.degree., although trim angle must be reduced as speed increases, to prevent porpoising. To achieve this optimum, a boat with a single running surface must be balanced with its center of gravity in a position fore-aft such that the boat is hydrodynamically balanced on the running surface at the desired trim angle. As boat speed increases, the immersed, roughly triangular, lifting portion of the hull must be smaller if trim angle is to be maintained. This requires that the center of forces acting on the boat must move aft to maintain the balance on the now smaller more rearward planing surface. Unstepped rigid-hull boats achieve this shifting of the center of force, to some extent, by tilting the drive unit to change the angle of the thrust vector. If the boat is to operate over a wide speed range, the drive tilt angle required to maintain boat trim becomes large and leads to loss of the forward thrust component, and increased drag.
An active hull design, controllably varying the planing surface configuration, allows the boat to achieve near optimum trim angles over a wide range of boat speeds. In one embodiment, the boat bottom is stepped forward of the transom a distance such that the boat trim angle at top speed is just below the porpoising limit. At lower speeds, the boat sinks farther into the water, causing the running surface to extend further forward, and the center of lift to move forward, increasing the trim angle, leading to porposing, higher drag, and obscured visibility at very low planing speeds, near the hump speed.
In one embodiment, active boat hull bottom trim tabs are provided in notches predominantly behind steps with lower surfaces offset upwards from the lower edges of the steps, and hinged so that they can be retracted upwards from and also depressed below a respective plane parallel to the boat bottom. In the retracted position, used at high speed, the tab remains clear of the water flow which separates from the step and is completely ventilated from the rear. At lower speeds, the tab is pivoted downwardly to an angle such that it produces the required amount of lift to reduce the boat trim angle to the desired value.
In one embodiment, a further enhancement is achieved by angling the pivot axis hinge line forwardly towards the chine. The step may or may not also be so angled. The angled hinge line causes the trailing edge of the tabs to move downwardly more on their outboard end than on their inboard end, effectively reducing the deadrise angle of the hull, and maintaining a running surface trailing edge free from discontinuities in the vertical direction. This has been beneficial for shaping the wake, improving skiing, wakeboarding, and the like.
In one aspect, the tabs can be operated differentially to control the roll angle of the boat, both straight ahead, and in turns, and to enhance steering. The steering enhancement is particularly valuable on boats with jet-type drives which do not have good off-throttle steering characteristics.